A new candidate pulsating ULX in NGC 7793

Abstract

We report here the discovery of NGC 7793 ULX-4, a new transient ultraluminous X-ray source (ULX) in NGC 7793, a spiral galaxy already well known for harbouring several ULXs. This new source underwent an outburst in 2012, when it was detected by \textit{XMM-Newton} and the \textit{Swift} X-ray telescope. The outburst reached a peak luminosity of 3.4$\times 10^{39}$ erg\ s${}^{-1}$ and lasted for about 8 months, after which the source went below a luminosity of $10^{37}$ erg\ s${}^{-1}$; previous \textit{Chandra} observations constrain the low-state luminosity below $\sim$ 2$\times 10^{36}$ erg\ s${}^{-1}$, implying a variability of at least a factor 1000. We propose four possible optical counterparts, found in archival HST observations of the galaxy. A pulsation in the \textit{XMM-Newton} signal was found at 2.52 Hz, with a significance of $\sim 3.4\sigma$, and an associated spin-up of $\dot{f}=3.5\times 10^{-8}$ Hz.s${}^{-1}$. NGC 7793 is therefore the first galaxy to host more than one pulsating ULX.

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